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SSL/TLS OpenVPN with HMAC Authenication

These instructions work on CentOS 6.x, Debian 6.x, Knoppix 6.x and probably other Linux distributions. (Easy-RSA and files kept in the directory /usr/share/doc aren't always available.) How the OpenVPN service runs on the server depends on how the service is configured to accept connections from clients. Additionally, clients have to be configured to communicate with that specific service. 

Server Instructions
1. Generate a RSA private key of 1024 bits encrypted using triple DES:
openssl genrsa -des3 -out ca.key 1024;

2. Generate a new certificate signing request using your RSA private key:
openssl req -new -key ca.key -out ca.csr;

3. Generate a self-signed root certificate that expires in 365 days:
openssl x509 -req -days 365 -in ca.csr -signkey ca.key -out ca.crt;
cp ca.crt /etc/openvpn/keys;
scp ca.crt root@<client hostname>:/etc/openvpn/keys;

4. Generate a certificate file and a key file for the server, sign it with the root certificate key:
openssl req -new -keyout server.key -out server.csr;
openssl x509 -req -days 90 -in server.csr -signkey ca.key -out server.crt;
mv server.key server.crt /etc/openvpn/keys;

5. Generate a certificate file and a key file for each client, sign it with the root certificate key:
openssl req -new -keyout client.key -out client.csr;
openssl x509 -req -days 30 -in client.csr -signkey ca.key -out client.crt;
scp client.crt client.key root@<client hostname>:/etc/openvpn/keys; 

6. Generate a static key to work with SSL/TLS certificates for HMAC authenication:
openvpn --genkey --secret hmac.key
cp hmac.key /etc/openvpn/keys;
scp hmac.key root@<client hostname>:/etc/openvpn/keys;

7. Create the configuration file "server.conf" of the OpenVPN service in the directory "/etc/openvpn":
touch /etc/openvpn/server.conf;
echo "local <private address>
port 1194
proto udp
dev tun0
ca /etc/openvpn/keys/ca.crt
cert /etc/openvpn/keys/server.crt
key /etc/openvpn/keys/server.key
secret /etc/openvpn/keys/hmac.key
server <network id of private network> <subnet mask>
keepalive 10 120
tls-auth hmac.key 0
comp-lzo
max-clients 5
user <user owner of tunnel>
group <group owner of tunnel>
status /var/log/openvpn/openvpn-status.log
verb 4
mute 10" > /etc/openvpn/server.conf;

8. Create the configuration file "tun0.conf" for the tun device of the OpenVPN service in the directory "/etc/openvpn":
touch /etc/openvpn/tun0.conf;
echo "dev tun0
ifconfig <server ip address> <client ip address>
secret /etc/openvpn/keys/hmac.key" > /etc/openvpn/tun0.conf; 

9. Enable kernel IP forwarding in the server:
echo '1' > /proc/sys/net/ipv4/ip_forward;
sysctl net.ipv4.ip_forward=1;

10. Automatically start your OpenVPN service on boot up:
update-rc.d openvpn <options>;

11. Edit the file "/etc/hosts.allow" that's the hosts access control list for allowing access to services on your server from specific hostnames, IP addresses, networks, and FQDNs:
<service or wildcard>: <hostname> <ip address>/<subnet mask> <fqdn>

12. Edit the file "/etc/hosts.deny" that's the hosts access control list for denying access to services on your server from specific hostnames, IP addresses, networks, and FQDNs:
<service or wildcard>: <hostname> <ip address>/<subnet mask> <fqdn>

13. Start the OpenVPN service:
service openvpn start;

14. Forward client connections to your OpenVPN service through your routed IP tunnel device "tun0":
iptables -A INPUT -i <interface> -p udp --dport 1194 -j FORWARD;
iptables -A FORWARD -i <interface> -o tun0 -m state --state ESTABLISHED,RELATED -j ACCEPT;
iptables -A FORWARD -o <interface> -s <private network>/<prefix length> -j ACCEPT;
iptables -t nat -A POSTROUTING -o <interface> -s <private network>/<prefix length> -j MASQUERADE;  

Client Instructions
1. Create the configuration file "client.conf" of the OpenVPN service in the directory "/etc/openvpn":
touch /etc/openvpn/client.conf;
echo "client
dev tun0
proto udp
ns-cert-type server
remote <server address> 1194
resolve-retry infinite
nobind
ca /etc/openvpn/keys/ca.crt
cert /etc/openvpn/keys/client.crt
key /etc/openvpn/keys/client.key
secret /etc/openvpn/keys/hmac.key
comp-lzo
verb 4" > /etc/openvpn/client.conf;

2. Create the configuration file "tun0.conf" for the tun device of the OpenVPN service in the directory "/etc/openvpn":
touch /etc/openvpn/tun0.conf;
echo "dev tun0
ifconfig <client ip address> <server ip address>
secret /etc/openvpn/keys/hmac.key" > /etc/openvpn/tun0.conf;

3. Check the client's configuration file for errors:
openvpn --config client.conf;

4. Add new routes to the OpenVPN service on the client:
ip route add <server ip address> via <gateway ip address> dev <outgoing interface> proto static;
ip route change default via <client tun0 ip address> dev tun0 proto static;

5. Allow outgoing client connections to the OpenVPN through your firewall:
iptables -A INPUT -i <interface> -p tcp --sport 1194 -j ACCEPT
iptables -A OUTPUT -o <interface> -p tcp --dport 1194 -j ACCEPT 

Do you have a suggestion about how to improve this blog? Let's talk about it. Contact me at David.Brenner.Jr@Gmail.com or 720-584-5229.

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